Neodymium oxide-barium oxide catalyst composition

ABSTRACT

The oxides of nitrogen content of a gas stream can be reduced by contacting the gas stream with a catalyst consisting essentially of neodymium oxide and barium oxide on a suitable support at elevated temperatures. The catalysts are especially useful in reducing the nitric oxide content of the exhaust gas of internal combustion engines.

l lnited States Patent Stephens 14s 1 July 1 1, 1972 {54] NEODYMIUM OXIDE-BARIUM OXIDE [56] References Cited CATALYST COMPOSITION UNITED STATES PATENTS I [72] Ruth E 3,459,494 8/1969 Hams et a1. ..23/2 3 [73] Assignee: Ethyl Corporation, New York, NY. 3,483,138 12/1969 Stephens ..252/462 [22] Filed: Aug. 31, 1970 Primary Examiner n E wyman [2 1] Appl, No.: 68,574 Assistant Examiner-W. J. Shine Altorney-Donald L. Johnson Related [1.8. Application Data [63] Continuation-in-part of Ser. No. 692,687, Dec. 22, [571 ABSWCT 19671 abandoned- The oxides of nitrogen content of a gas stream can be reduced by contacting the gas stream with a catalyst consisting essen- [52] US. Cl. ..252/455 R, 252/457, 252/462, tiauy f neodymium oxide and barium id on a i bl 252/475 port at elevated temperatures. The catalysts are especially 511 lm. c1. .301 11/00, B01 11/06, 801; 11/40 f in reducing the nitric oxide come, f the ex as 581 Field of Search ..252/455 R, 457, 462, 475; f internal combustion engines 5 Claims, No Drawings NEODYMIUM OXlDE-BARIUM OXIDE CATALYST COMPOSITION CROSS REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of application, Ser. No. 692,687, filed Dec. 22, 1967, now abandoned.

BACKGROUND The exhaust gas of internal combustion engines contains unburned hydrocarbons, carbon monoxide, nitrogen oxides and oxygen, among other materials. Investigators have reported that nitrogen oxidesin the presence of sunlight lead to ozone formation and that ozone reacts with hydrocarbon substituents in the atmosphere to form noxious materials. Therefore, extensive research has been carried out directed at means of removing the precursor materials, nitrogen oxides and hydrocarbons, from the atmosphere. The present invention describes a catalyst, a method of using this catalyst to reduce the oxides of nitrogen content of the atmosphere by contacting exhaust gas of internal combustion engines or other exhaust gases containing oxides of nitrogen with a catalyst.

SUMMARY This invention relates to a catalyst eminently useful for decomposing oxides of nitrogen. In particular, this invention relates to a catalyst consisting essentially of neodymium oxide and barium oxide impregnated on a suitable support and to the use of this catalyst in decomposing oxides of nitrogen.

An object of this invention is to provide a catalyst useful for decomposing nitrogen oxides. The catalyst can be used in reducing the nitrogen oxide content of the exhaust gas of internal combustion engines or in reducing the nitrogen oxide content of other effluent exhaust streams such as that emitted during the manufacture of nitric acid from ammonia.

The objects of this invention are accomplished by providing a catalyst consisting essentially of from 0.001 to weight percent neodymium in an oxide form and from 0.0001 to 15 weight percent barium in an oxide form impregnated on a suitable support, the catalyst having an atom ratio of barium to neodymium offrom 1:10 to 1:100.

In a preferred embodiment, the catalyst consists essentially of from 0.001 to 15 weight percent neodymium in an oxide form and from 0.001 to 15 weight percent barium in an oxide form impregnated on a support selected from the group consisting of zirconia,alumina-zirconia, calcium aluminate, alumina-titania, alumina, magnesia, alumina-magnesia, silica, silicaalumina, silica-magnesia, silicon carbide, zinc oxide, mullite, synthetic and natural zeolites, silica-zirconia, kaolin, silica-titania, magnesia-zirconia, magnesia-titania, zirconiatitania, alumina-silica-magnesia, alumina-silica-zirconia, alumina-silica-titania, alumina-magnesia-zirconia, alumina-magnesia-titania, alumina-zirconia-titania, silica-magnesia-zirconia, alumina-magnesia-titania, silica-zirconia-titania, magnesia-zirconia-titania, or mixtures of these support materials.

In a highly preferred embodiment, the support material comprises a substantial amount of zirconia. By a substantial amount is meant above about 50 percent. The balance may be other typical support materials. As especially useful support is zircon, which contains about 60 percent zirconia and 30 percent silica, the balance being mainly alumina.

The catalyst can be prepared by any one of many methods known to catalyst manufactures. A suitable support can be merely impregnated with a solution of a soluble salt of neodymium and barium. Examples of such salts include neodymium nitrate, neodymium acetate, barium nitrate and barium acetate. After impregnation, the support is calcined to decompose the salts of neodymium and barium to an oxide form. Another method is to mix neodymium and barium oxide with a precursor of the support material. For example, an alumina gel can be formed by adding sodium hydroxide to an acidic solution of an aluminum salt such as aluminum nitrate dissolved in aqueous nitric acid. Neodymium oxide and barium oxide can be mixed with this gel is the proper amount, forming a paste which is then extruded, dried, and calcined, resulting in a suitable catalyst. Likewise, other support materials may be included in the above process such as kaolin, silica and zirconia. In a' similar manner, the neodymium and barium salts may be in solution with a water soluble aluminum salt such as aluminum nitrate in dilute nitric acid and coprecipitated with the alumina gel formed when base is added. The resulting gel can be extruded, dried and calcined or it may be mixed with other dry support material such as kaolin, silica, and the like, forming a paste which can then be extruded, dried and calcined.

The following examples will serve to illustrate several of the methods available for preparing the barium oxide promoted neodymium oxide catalysts of this invention. All parts are by weight unless otherwise specified.

EXAMPLE 1 In an open vessel was placed 6 parts of water, 4.38 parts (0.01 mole part) of neodymium nitrate hexahydrate and 0.05 part (.00025 mole part) of barium nitrate. After a solution formed, 9.25 parts of zircon, a catalyst support comprising a substantial amount of zirconia (Zircon-Norton L28 4045 three-sixteenths-inch spheres), was added and the mixture allowed to stand until the solution had impregnated the support. The impregnated zirconia support was then calcined at 700 C. for an hour, resulting in a catalyst containing 12.3 weight percent neodymium and 0.36 weight percent barium, both in oxide forms, on a support comprising a substantial amount of zirconia.

EXAMPLE 2 In the vessel of Example 1 place 438 parts of neodymium nitrate hexahydrate, 2.61 parts of barium nitrate, and 500 parts of water. Mix until dissolved and then add 730 parts of alumina pellets (primarily gamma-alumina). Let the mixture stand until the solution impregnates the alumina. When the aqueous phase is almost gone, warm the mixture to C. to evaporate off excess water and then calcine the impregnated alumina at 700 C. for 2 hours. The catalyst formed consists essentially of 15 percent neodymium and 0.15 percent barium, both in oxide forms, on an alumina support.

EXAMPLE 3 In the mixing vessel of Example 1 place 448 parts of neodymium nitrate hexahydrate, 2,610 parts of barium nitrate, and 4,000 parts of water containing 10 parts of HNO To this add 7,500 parts of 6-10 mesh zinc oxide pellets. Allow this to stand until the aqueous phase is substantially absorbed and then warm it to 100 to dry the catalyst. When dry, calcine the catalyst at 700 C. for 2 hours. The resulting catalyst contains 1.5 percent neodymium and 15 percent barium, both in an oxide form, impregnated on a zinc oxide support.

In the above example, other supports can be used with good results, such as zeolites, magnesia, silica, silica alumina, and the like.

EXAMPLE 4 A paste is made by digesting a mixture of 50 parts alumina gel and 40 parts of kaolin with 10 percent nitric acid. A mixture of 10 parts of neodymium oxide and 0.01 parts of barium oxide is mixed into the paste and the mass is extruded through a three-sixteenth inch die, forming pieces one-fourth inch long. The extrudate is dried at 100 C. and then slowly heated to 700 C. over a 4 hour period. The resulting catalyst contains 8.5 percent neodymium and 0.009 percent barium, on oxide forms.

In the foregoing examples the concentration of the neodymium and barium can be readily varied by merely using a different quantity of the neodymium or barium containing materials in relation to the amount of support. In another variation, the support may be impregnated with the neodymium and barium salts in separate operations. For example, a zirconia support may be first impregnated with 1 weight percent barium, as barium nitrate, followed by drying, and then with weight percent neodymium, as neodymium nitrate, followed by drying and calcination. If desired, this order can be reversed.

In order to be effective in decomposing oxides of nitrogen the catalysts of this invention must be heated to an elevated temperature. Generally the temperature must be above about 400 C. before the catalysts become active. A generally effective temperature range is from 400-1,000 C., and a preferred temperature range is from 700850 C.

Tests have been carried out to demonstrate the ability of the barium promoted neodymium oxide catalysts to catalyze the decomposition of oxides of nitrogen. In these tests, a stream of helium was passed through a heated catalyst bed and then through a vapor phase chromatograph. The vapor phase chromatograph was adjusted to baseline with the helium stream. A small amount of nitric oxide was then injected into the helium stream ahead of the catalyst bed. The vapor phase chromato graph then determined the composition of the gas stream after contacting the catalyst.

The above test was carried out on barium promoted neodymium oxide catalysts prepared in the manner of Example l. The atom ratio of barium to neodymium in the catalysts used in these tests was varied over a wide range. The amount of neodymium and barium added to the catalyst was such that a total of 0.01 gram atoms of metal was impregnated on each 14 grams of zircon support. The catalyst was maintained at 750 C. In all cases, the catalysts effectively decomposed the nitric oxide into nitrogen and oxygen. The results of these tests are shown in the following table in which the chromatographic response is expressed in arbitrary units.

Chromatographic Response From the above results it is seen that unpromoted neodymium oxide catalyzed the decomposition of nitric oxide to a moderate extent. However, when barium was added in only 1 atom part per 100 atom part of neodymium, the effectiveness of the neodymium was greatly increased. The amount of nitrogen in the exit gas formed by the decomposition of nitric oxide increased from 960 units to 1,224 units a 27.5 percent increase. Therefore, a preferred atom ratio of barium to neodymium is from 1:10 to 1:100. The promoter effect of barium was observed at all atom ratios. When the catalyst contained only barium oxide and no neodymium oxide the exit gas was almost unchanged; only 32 units of nitrogen formed and the amount of nitric oxide increased from only trace amounts to 2,380 units.

When using the catalyst of this invention to decompose the nitrogen oxides in the exhaust stream of an internal combustion engine it is preferred that the engine be operated at an air/fuel ratio below about 14 in order to obtain maximum effectiveness. At leaner ratios the large amount of oxygen in the exhaust stream reduces the ability of the catalyst to decompose nitrogen oxides.

In order to use the catalyst in an internal combustion engine exhaust system, the catalyst is incorporated in a suitable manner into the exhaust system of the engine. One method commonly used is to place the catalyst in a so-called "catalytic muffler." Examples of these are disclosed in U.S. Pat. Nos. 3,154,389; 3,] 9,925; 3,149,926 and 3,146,073, among others. Essentially these are containers having an opening to receive and discharge the exhaust gas. To firmly retain the catalyst material, the receiving and discharge openings are covered with wire screen. The container may have internal baffling to allow greater contact between catalyst and exhaust gas or to use the hot reaction gases to heat the incoming exhaust gases. The container may actually replace the vehicle muffler or may be incorporated into the conventional exhaust system of current vehicles. The catalyst bed may also be located in the exhaust manifold of the engine.

The barium oxide promoted neodymium oxide catalysts may be used by themselves, as mixtures, or they may be used in conjunction with a second catalyst whose function is to oxidize the hydrocarbon or carbon monoxide constituents of the exhaust gas. A catalyst eminently suited for this purpose is a supported copper-palladium catalyst as described in U.S. Pat. No. 3,224,981. The barium oxide promoted neodymium oxide catalysts may be intimately mixed with the oxidation catalyst or the different catalysts may be stratified.

When used to decompose oxides of nitrogen in streams other than the exhaust stream of internal combustion engines, the catalyst is merely incorporated in the oxide of nitrogen containing stream so that intimate contact is obtained between the catalyst and the oxides of nitrogen. For example, in the discharge stream of a nitric acid plant employing the ammonia process for synthesizing nitric acid, the spent gas containing nitric oxide is passed through the barium oxide promoted neodymium oxide catalyst bed and the temperature of the bed maintained at a temperature of from about 400 to 1 ,000 C.

lclaim:

l. A catalyst suitable for decomposing oxides of nitrogen, said catalyst consisting essentially of 0.001 to 15 weight percent neodymium in an oxide form and from 0.0001 to 15 weight percent barium in an oxide form impregnated on a suitable support, said catalyst having an atom ratio of barium to neodymium offrom 1:10 to 1:100.

2. The catalyst of claim 1 wherein said support comprises a substantial amount of zirconia.

3. The catalyst of claim 2 wherein said neodymium is present in an amount from about 5 to 10 weight percent in an oxide form and said barium is present in an amount from about 0.5 to 3 weight percent in an oxide form.

4. The catalyst of claim 2 wherein said support is zircon, said zircon containing about 60 percent zirconia, 30 percent silica, and the balance alumina.

5. The catalyst of claim 2 wherein said neodymium is present in an amount of about 12.3 weight percent in an oxide form and said barium is present in an amount of about 0.36 percent in an oxide form, based on the total weight of said catalyst. 

2. The catalyst of claim 1 wherein said support comprises a substantial amount of zirconia.
 3. The catalyst of claim 2 wherein said neodymium is present in an amount from about 5 to 10 weight percent in an oxide form and said barium is present in an amount from about 0.5 to 3 weight percent in an oxide form.
 4. The catalyst of claim 2 wherein said support is zircon, said zircon containing about 60 percent zirconia, 30 percent silica, and the balance alumina.
 5. The catalyst of claim 2 wherein said neodymium is present in an amount of about 12.3 weight percent in an oxide form and said barium is present in an amount of about 0.36 percent in an oxide form, based on the total weight of said catalyst. 